The present invention relates to vehicle safety systems and, in particular, it concerns a proximity warning system for warning of the proximity of an obstacle adjacent to a vehicle.
A large number of systems have been suggested for detecting and warning a driver of the presence of stationary or moving obstacles adjacent to a vehicle, typically, a motor vehicle.
Of most relevance to the present invention are a number of examples of active optical systems in which a signal, typically infrared, is transmitted towards the zone of interest and the reflected signals are analyzed. Examples of such systems are described in U.S. Pat. No. 4,825,211 to Park, U.S. Pat. No. 5,122,796 to Beggs et al. and U.S. Pat. No. 5,463,384 to Juds.
Practical implementations of the aforementioned systems a plagued by a number of problems as will now be described with reference to FIGS. 1-3. FIG. 1 illustrates the output current of a typical photodiode as a function of incident radiant energy, while FIG. 2 shows the corresponding variation in sensitivity. In order for the photodiode to remain functional even when exposed to direct sunlight, extensive filtering must be used to avoid saturation. Even after such filtering, the sensitivity of the photodiode under high levels of background radiation is severely reduced. As a result, high energy levels of transmitted signal are required to allow effective measurement.
FIG. 3, on the other hand, shows the relationship between optical output power and the lifetime for a typical LED suitable for use in these systems. It may be seen that high power use drastically reduces the lifetime of the LED compared with lower power usage. Thus, the high power required for the system to be functional under strong illumination conditions typically leads to rapid failure and unreliability of the transmitter elements.
A further shortcoming of the existing systems is their inability to provide complete reliable coverage of a well-defined zone adjacent to the vehicle. Thus, by way of example, Beggs et al. monitors various circular segments whereas Juds speckles a rectangular region with various isolated beams. For applications such as reversing, a driver needs to be made aware of any obstacle present within a given distance from the rear of the vehicle, independent of the transverse position of the obstacle and without risk of it falling "between" the detection beams. None of the aforementioned systems can satisfy such requirements.
Finally, there remains a need for a manner of providing unambiguous information to the driver of the vehicle as to the proximity of an obstacle. Certain systems have provided a variable pitch warning in which the pitch of an audible signal varies as a function of the distance to an obstacle. However, such a warning is open to subjective interpretation.
There is therefore a need for a proximity warning system for vehicles which can operate effectively in any illumination conditions while maximizing the lifetime of its components. It would also be advantageous to provide a proximity warning system for vehicles which provides complete reliable coverage of a well-defined zone adjacent to the vehicle, and gives an unambiguous indication to the driver of the proximity of an obstacle.